1. Field of the invention
The present invention relates to a resistor integrated on a semiconductor substrate and more especially a resistor having one of its ends connected to a point of the substrate subject to a high potential.
In numerous applications concerning semiconductor structures associating a power transistor and its control and/or protection logic, further designated "smart power switches", it is desirable to have resistors connected to the drain or to the collector of the power transistor. If the power transistor is of the VD MOS or bipolar type with vertical conduction, the rear face of the substrate supporting the drain or collector contact is therefore at the potential of this drain or this collector. An electronic ignition circuit for motor vehicles constructed in monolithic form may require a resistance voltage divider bridge withstanding a voltage of the order of 400 V for re-enabling the power transistor in the case of an over voltage. The construction of such a resistor comes up against serious insulating difficulties if it is desired to avoid, in the manufacturing process, certain special steps intended to provide insulation by means of the physical screen.
2. Description of the Prior Art
Generally, different methods are known for integrating a resistor on a semiconductor substrate of given polarity. The natural resistance of an epitaxial layer may be used but this leads to resistance values which are difficult to control and does not allow divider bridges to be formed. This resistor may also be formed by means of a diffused caisson of the opposite opposite conductivity type relative to the substrate. although this solution provides precise control of the value of the resistance, on the other hand it requires the use of special steps for ensuring its physical insulation and these steps are undesirable. Without this physical insulation, such a resistor would, in the case of a device operating at high voltage (for example 400 V), cause breakdown of the junction existing between the substrate and the caisson which is due to the excessive curvature of the equipotentials. In fact, the connection of the resistor thus formed to the rear face of the substrate brought to the potential of the high voltage involves taking the equipotentials up to the front face of the substrate and some of these equipotentials have very small radius of curvature.
In order to overcome these drawbacks, the invention provides an integrated resistance formed by a well of polarity the reverse of that of the substrate, in the form of a ribbon would about one of its ends which is attached to a conducting pole. This conducting pole provides the electrical connection with the rear face of the substrate therethrough. The ribbon serves both as resistor and field distributor rings, which avoids the excessive curvature of the equipotentials.